Mirror Modifier does not work on where ther are no faces
All modifiers have several options that alter the way they are displayed. Not all modifiers have every option available. The buttons for these display choices are shown at the top of each modifier -



The first two options  are available to all modifiers and make the modifier visible in final render or viewport display. In object mode in the viewport with this enabled you will see the edges created by the mirror modifier.



When available the other two options effect what modifier data is shown in edit mode.

While in edit mode and with the third option disabled you will not see any of the mesh created by the modifier.



While in edit mode and the third option enabled you will see only faces drawn that are created by the modifier. This is the default for many modifiers.



The final option allows the full edit mesh to be displayed. With this enabled you can select parts of the mesh on the mirrored side. Be careful with this option as all movements are done on the real geometry only, as you select mirrored vertices and move them they will move 'backwards' to what you expect.



Also remember that with the mirror modifier, if your mesh is centred around the point of the mirror, the duplicated mesh may be located in the same place as existing mesh data.